#603 - drawImage() keywords for setting output size.

This PR introduces a few convenience keywords to drawImage() to set the output image size, either by resizing an existing image or by overriding the `getGoodImageSize` values in the case where a new image gets constructed.

Here's an excerpt from the updated docstring:

```
        @param nx           If provided, use to set the x-direction size of the image.  Must be
                            accompanied by `ny`.
        @param ny           If provided, use to set the y-direction size of the image.  Must be
                            accompanied by `nx`.
        @param bounds       If provided, use to set the bounds of the image.
```

There are also a few new unit tests to go along with these.
And if you put an intermediate line in, then it does change the scale:

```
foo = galsim.Image(32,32,scale=1.7)
new_bounds = galsim.BoundsI(5,7,14,29)
bar = foo[new_bounds]
bar.scale = 2.0
assert bar.scale == 2.0
```